1. Field of the Invention
This invention relates to the synchronization of digital transmission systems and, more particularly, to the synchronization of digital transmission terminals including digital concentrator terminals therebetween.
2. Description of the Prior Art
When a digital concentrator system is used with a digital carrier system, the concentrator terminals may conveniently be inserted between the carrier terminals. Such a concentrator system is disclosed in applicant's copending application Ser. No. 186,617, filed of even date herewith.
In such a system, a plurality of digitally encoded and multiplexed bit streams are intercepted at a concentrator terminal which performs either concentration or expansion. It is necessary for the terminals of such a system to be synchronized in three different senses: first, the framing bits must be transmitted between the concentrator terminals in the exact same positions in which they are retrieved from the digital carrier terminals so that the carrier terminals themselves can achieve framing; second, each bit position at the output of the concentrator terminal must be identified to permit demultiplexing; third, in order to permit the proper transmission of signaling bits through the concentrator terminal at a multiframe rate, the frame bit must be associated with the same frame from which they were derived. These three requirements create serious problems when synchronization is lost since the proper framing bit positions may be relocated in the course of the expansion function.